The invention relates to a sheet of paper, especially a sheet of absorbent paper made of creped tissue, having an embossed pattern.
The invention relates in particular to a sheet of paper composed of at least a first and a second ply of creped tissue, intended for the field of wiping in a household, domestic or industrial application, that is to say capable of being used as kitchen roll, or towel, for example.
In the field of paper for sanitary or domestic use such as toilet paper or kitchen roll, it is known to produce sheets of paper composed of several plies of absorbent paper, for example made of creped tissue having a basis weight between 12 and 30 g/m2, provided with protrusions that are obtained by embossing.
This is because embossing gives bulk to the sheet and it improves the absorption of liquids, the feel and the softness. Attempts have been made to increase the absorption capacity still further by creating multilayer sheets which are obtained by combining at least two layers, also known as plies, each consisting of at least one of said embossed sheets.
Thus, a multilayer (or multiply) sheet is obtained which has given mechanical properties such as tensile strength and also absorption capacity.
Two methods of embossing and assembling the plies are commonly used depending on the desired properties of the end product.
The first is known in the field by the name “nested”. It consists firstly in embossing each of the plies separately so as to form, at the surface, protrusions which are generally substantially frustoconical or in the form of pyramid frustums. Next, adhesive is applied to the top of the protrusions of one of the plies, and the plies are arranged so that the faces having the protrusions face each other, the protrusions of one being between the protrusions of the other. Finally, the plies are assembled so that the protrusions that have been coated with adhesive nest between the protrusions of the other ply. Thus, the two plies are joined together by spots of adhesive between the tops of the protrusions of one ply and the unembossed regions between the protrusions of the other ply. A structure is produced in which the voids thus formed are able to give the sheet better absorption. Furthermore, the outer faces have a smooth and soft feel due to the hollow regions formed by the back of the protrusions. This technique is illustrated, for example, by Patent U.S. Pat. No. 3,867,225.
The second method of assembly is known in the field under the name “point-to-point”. It differs from the preceding method by the relative arrangement of the two plies. These, after they have been separately embossed, are brought together in such a way as to make the tops of the protrusions, or at least part of these tops, coincide. The plies are joined together by the tops of the protrusions, point-to-point. This technique is illustrated by Patent U.S. Pat. No. 3,414,459.
When the practical realizations of these structures are considered, whether they are of the nested or point-to-point type, the protrusions obtained during embossing are usually produced so as to be distributed in a pattern in which the protrusions are generally aligned in one or more directions, uniformly. Such a pattern makes it possible to obtain a sheet that is homogeneous as regards its mechanical properties.
However, in order to improve user satisfaction, it is important to increase the flexibility, also called “the bulk” of the sheet.
The flexibility of the sheet can be defined as being its ability to deform uniformly when it is placed in the hand of a user, their fingers being stretched out as an extension of their palm and when the user then closes their fingers.
A sheet of paper which has a high flexibility will then deform and bend or curve so as to remain substantially parallel to the shape determined by the inside of the palm of the user's hand.
A sheet of paper that has poor flexibility will fold along one or more lines, each portion of the sheet which extends between two fold lines remaining substantially flat.
A sheet of paper that has high flexibility is more pleasant to use.
Also known is an embossed multiply sheet such as described in Patent Application EP 1 236 830 A1 in which at least one first ply has a first embossed pattern consisting of a first series of first protrusions of frustoconical overall shape projecting from the inner face of the first ply, which in particular form first alignments and the tops of which are joined to the inner face of the second ply opposite each other; the two plies each have a second embossed pattern made up of a second series of second protrusions of frustoconical overall shape projecting from the inner face of the corresponding ply, and in that the tops of the second protrusions of one ply extend opposite and near the tops of the second protrusions of the other ply.